An optically functional surface of a lens for use in a camera or the like is sequentially processed by taking the steps of rough machining of a glass raw material, rough grinding, polishing, and fine polishing, with predetermined surface accuracy established for each step.
In these steps, a processing position in which a processing tool comes into contact on a lens surface to be processed and a surrounding area of the processing position are thoroughly supplied with polishing liquid including polishing abrasive grains.
Many types of lens polishing apparatuses have been developed in the past and have been used with varying degrees of success. These apparatus are believed to belong to two broad groups. The first group utilizes a lapping tool head having a resilient or flexible lapping membrane that is deformable upon contact with the surface of a lens to adapt to the curvature of the lens. Examples of apparatuses belonging to this first group are described in the following United States patents: U.S. Pat. No. 3,589,071 issued on Jun. 29, 1971 to Hans S. Hirshhorn; U.S. Pat. No. 5,205,083 issued on Apr. 27, 1993 to Dennis R. Pettibone; and U.S. Pat. No. 5,662,518 issued on Sep. 2, 1997 to Michael D. James et al. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.
The second type of lens polishing apparatus of the prior art uses a plurality of plungers for applying pressure gradients over a lens polishing membrane. Although this apparatus is designed for polishing large telescope mirrors, this is the type of apparatus that is of interest herein. Examples of these apparatuses are illustrated in the following United States patents: U.S. Pat. No. 4,606,151 issued on Aug. 19, 1986 to Erich Heynacher; and U.S. Pat. No. 4,802,309 issued on Feb. 7, 1989 to Erich Heynacher. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.
Another type of polishing apparatus that utilizes vibrational motion was developed mainly for polishing metals and utilizes a tool that only becomes conformal to the surface being processed after vibration occurs. This type of vibrational polishing method may work for rapid removal but does not deal with pre-existing signatures in the material. This type of apparatus is employed to polish any material more resistant to vibrational (sonic and ultrasonic) erosion than the material of which the tool is made. In this fashion, the tool will be re-dressed continuously and inherently to the complementary form of the work piece, by virtue of the fact that the tool will be eroded to a greater extent than the work piece. The preferential working of the tool results in a constant or even increasing conformity to the fine detail and resolution of the work piece, so that, as polishing of the work piece occurs, there is no loss of resolution. Examples of these apparatuses are illustrated in U.S. Pat. No. 5,187,899 issued on Feb. 23, 1993 to Lawrence J. Rhoades. The entire disclosure of this United States patent is hereby incorporated by reference into this specification.
Other types of related polishing apparatuses for use with quartz, glass, ceramics, metals, and plastics have applied the ultrasonic energy directly to the substrate. While using a lapping tool over the surface being processed, the result is a half wave resonance at that first surface. An example of these apparatuses is illustrated in U.S. Patent: U.S. Pat. No. 5,551,907 issued on Sep. 3, 1996 to Dave Sandeep. The entire disclosure of this United States patent is hereby incorporated by reference into this specification.
After a surface has been ground to a specific form, that form can be monitored throughout the manufacturing process by periodically monitoring roughness values such as the Rt or Ra of the surface.
Rt is a common measure of roughness used in the abrasives industry; however, the exact measuring procedure can vary with the type of equipment utilized in surface roughness evaluation. Rt measurements are based on procedures followed with the Rank Taylor Hobson profilometer located in Leicester, England, available under the trade designation “SURTRONIC 3.” Within the Rank Taylor Hobson purview, Rt is defined as the maximum peak-to-valley height within an assessment length set by the Rank Taylor Hobson instrument. Rt is the average, measured over five consecutive assessment lengths, of the maximum peak-to-valley height in each assessment length. Rt is measured with a profilometer probe which, for the SURTRONIC 3, is a 5 micrometer radium diamond tipped stylus; and the results are recorded in micrometers (um). For a further discussion of such “Rt” term, reference maybe had, e.g., to U.S. Pat. Nos. 5,873,770, 5,888,119, 6,110,015, 6,194,317, 6,231,629, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.
Ra is defined as an average roughness height value of an arithmetic average of the departures of the surface roughness profile from a mean line on the surface, also measured in micrometers (um). Reference may be had, e.g., to U.S. Pat. Nos. 5,876,268, 5,989,111, 6,042,928, 6,086,977, 6,155,910, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.
After a surface has been rough ground, it is polished and then fine polished. For glass-surface polishing, it is preferred that average particle size of the abrasive particles is from about 0.001 to 20 micrometers, typically between 0.01 to 10 micrometers. In some instances, the abrasive particles preferably have an average particle size less than 0.1 micrometer. In other instances, it is preferred that the particle size distribution results in no or relatively few abrasive particles that have a particle size greater than about 2 micrometers, preferably less than about 1 micrometer and more preferably less than about 0.75 micrometers. At these relatively small particle sizes, the abrasive particles may tend to aggregate by interparticle attraction forces. Thus, these aggregates may have a particle size greater than about 1 or 2 micrometers and even as high as 5 or 10 micrometers. It is then preferred to break up these aggregates to an average size of about 2 micrometers or less.
During processing, there are many machines that utilize tools that make a point or localized contact with the surface being processed.
The manner in which these machines remove material from the surface being processed causes signatures on that surface that are difficult and time consuming to thoroughly remove with conventional methods.
As used in this specification, the term “signature,” refers to irregularities in the surface of the substrate caused by prior operations. Reference may be had, e.g., to U.S. Pat. Nos. 6,039,630, 6,396,995, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.
As will be apparent to those skilled in the art, each particular operation tends to leave its own distinct signature. The presence of these signatures is undesirable, for they often affect the optical properties of the substrate.
Signatures left on and in the surface being processed up to and including the polishing step vary dramatically based on a number of variables, however, two general problems can be stated. In the case of rotationally symmetric aspheric surfaces, fine grinding, can leave subsurface damage, which results in longer polishing cycles. Point contact of tooling to the surface being processed in conjunction with opposed rotational motion commonly associated with the manufacturing of rotationally symmetric, aspheric lenses, results in sometimes broken annular rings (from a top view) that exist in the form of a wave pattern on the surface being processed. This wave pattern commonly referred to as midspatial roughness, ranges in wavelength from above 80 micrometers up to about 8000 micrometers depending on a variety of factors. It is the object of one aspect of this invention that this wave pattern be minimized as much and as quickly as possible.
Numerous attempts have been made to decrease the time necessary to obtain the desired surface finish and extend abrasive pad life during the polishing process. For instance, U.S. Pat. No. 5,014,468 (Ravipati et al.) discloses a lapping film intended for ophthalmic applications comprising an interconnected pattern imparted into a surface coating of abrasive particles dispersed in a radiation cured adhesive binder. The entire disclosure of this United States patent is hereby incorporated by reference into this specification.
One object of the present invention is to remedy the shortcomings of related techniques and to eliminate a specific scope of surface and subsurface related signatures, and to do so in the shortest amount of time possible.